The invention relates to a liquid-crystalline medium and to twisted nematic (TN) and supertwisted nematic (STN) liquid crystal displays having very short switching times and good steepness and angular dependence and to the novel nematic liquid crystal mixtures employed therein.
TN displays are known, e.g. from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, e.g. from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Conference on Liquid Crystals (Aug.-Nov. 4, 1987); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, AppI. Phys. Lett. 50 (5), 236 (1987), and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term STN herein comprises any relatively strongly twisted display element having a twist angle amounting to between 160xc2x0 and 360xc2x0, for example the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), the STN LCDs (DE-A-35 03 259), SBE LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST LCDs (EP-A 0 246 842) or BW STN LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
STN displays in particular are distinguished, compared with standard TN displays, by considerably better slopes of the electro-optical characteristic curve and the attendant better contrast values and by a significantly reduced angular dependence of the contrast.
Of interest are TN and STN displays having a very short switching time, especially at lower temperatures. To achieve short switching times it has hitherto been the practice to optimize the rotational viscosities of the liquid crystal mixtures by employing usually monotropic additives having a relatively high vapor pressure. The switching times achieved were not, however, adequate for all applications.
To achieve a steep electro-optical characteristic curve in the displays according to the invention, the liquid crystal mixtures should have relatively large values of K33/K11 and relatively small values of xcex94xcex5/xcex5xe2x8axa5, xcex94xcex5 being the dielectric anisotropy and xcex5xe2x8axa5 being the dielectric constant perpendicular to the longitudinal axis of the molecules.
Beyond optimized contrast and optimized switching times, such mixtures are subject to further important requirements:
The parameter combinations achieved are still inadequate by a long shot, especially for high-multiplex STN displays (with a multiplex rate in the range of about 1/400), but also for low- and medium-multiplex STN displays (with multiplex rates in the range of from about 1/64 and 1/16, respectively), and TN displays. One reason for this is that the effects of material parameters on the various requirements pull in opposite directions.
There is therefore still a great need for TN and STN displays, especially for low- and medium-multiplex STN displays, having very short switching times in conjunction with a wide operating temperature range, a steep characteristic curve, good angular dependence of contrast and low threshold voltage, which comply with the above-specified requirements.
An object of the invention is to provide liquid-crystalline media, especially for TN and STN displays, which do not have the above-specified drawbacks or which have them only to a lesser extent and at the same time have short switching times, in particular at low temperatures, and have very good steepness.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects can be achieved by employing liquid-crystal mixtures which comprise one or more compounds of formula A 
and at least one compound of formula B 
where
Ra, Rb 
and Rbb are each, independently, an alkyl radical having 1 to 12 C atoms which is unsubstituted, singly substituted by CN or CF3 or at least singly substituted by halogen, with the additional option of one or more CH2 groups in the radicals, independently of one another in each case, being replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, 
xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Cxe2x80x94COxe2x80x94Cxe2x80x94 in such a way that O atoms are not directly linked together,
L1 is H or F,
Q is CF2, OCF2, CFH, OCFH or a single bond
Y is F or Cl, and
Qxe2x80x94Y alternatively also is NCS, SCN, OCHFCF3 or SF5.
The use of the compounds of formulae A and B in mixtures for TN and STN displays according to the invention results in
a steeply sloping electro-optical characteristic curve,
a low temperature dependence of the threshold voltage, and
very rapid switching times, especially at low temperatures.
The compounds of formula A and B, in particular, distinctly shorten the switching times of TN and STN mixtures, while at the same time result in an increase in the steepness and a low temperature dependence of the threshold voltage.
The mixtures according to the invention are further distinguished by the following advantages:
they have low viscosity,
they have a low threshold voltage and operating voltage,
they result in long storage times in the display at low temperatures.
The invention further relates to a liquid crystal display comprising
two substrates which, together with an edging, form a cell,
contained in the cell, a nematic liquid crystal mixture having positive dielectric anisotropy,
electrode layers having alignment layers on the insides of the substrates,
a pre-tilt angle between the longitudinal axis of the molecules at the surface of the substrates and the substrates of from 0 to 30 degrees, and
a twist angle of the liquid crystal mixture in the cell which, from alignment layer to alignment layer, is 22.5xc2x0 to 600xc2x0,
a nematic liquid crystal mixture comprising
characterized in that component A comprises at least one compound of formula A, 
and component B comprises at least one compound of formula B 
where
where Ra, Rb, Rbb, L1 and Q-Y have the meanings specified previously.
The invention also relates to TN and STN displays, especially in medium- and low-multiplexed STN displays, which contain the liquid-crystal mixture according to the invention.
In particular, formula A comprises compounds of the subformulae A-1 to A-8, 
where Ra and L1 have the meanings specified previously.
Preferred are mixtures according to the invention which comprise at least one compound of formula A-1 and/or A-2, more preferably at least one compound of formula A-1 where L1=F.
In the formulae A and A-1 to A-8, Ra preferably is straight-chain alkyl or alkoxy, or 1E-alkenyl or 3E-alkenyl having 2 to 7 C atoms.
Formula B comprises compounds of the subformulae B-1 to B-5, 
where alkyl or alkyl* each, independently of one another, is a straight-chain or branched alkyl radical having from 1 to 12 C atoms, and
alkenyl or alkenyl* each, independently of one another, is a straight-chain or aranched alkenyl radical having from 2 to 12 C atoms.
Preferably, the radicals xe2x80x9calkylxe2x80x9d or xe2x80x9calkyl*xe2x80x9d and xe2x80x9calkenylxe2x80x9d or xe2x80x9calkenyl*xe2x80x9d are straight-chain and have up to 7 C atoms.
The use of the compounds of formulae A and B in the liquid-crystal mixtures according to the invention results in particularly low values for the rotational viscosity in TN and STN displays having high steepness and rapid switching times, especially at low temperatures.
Component A and the liquid-crystalline mixture according to the invention, in addition to the compounds of formula A, preferably contain one or more 3,4,5-trifluorphenyl compounds selected from the compounds of the formulae IIa to IIk, 
In addition to the compounds of formulae A and B, the medium according to the invention can also contain one or more compounds having a polar terminal group of the formulae II*a to II*s, 
where R2 in formulae IIa-IIk and II*a-II*s has one of the meanings specified for Ra, and L3 and L4 each, independently of one another, is H or F. R2 in these compounds is preferably alkyl, alkenyl or alkoxy having up to 7 C atoms.
Preferably, the medium according to the invention or component A comprises compounds of the formulae IIa, IIb, IIc, IId, IIe, IIf, IIg, IIj, II*b, II*c, II*d, II*f and/or II*i, more preferably one or more compounds of the formulae IIa, IIb, IId, IIi, II*a and II*i.
The mixture according to the invention preferably contains one or more cyano compounds of the formulae IIIa to IIIj: 
where R3 has one of the meanings specified for Ra, and L1, L2 and L5 are each, independently of one another, is H or F. R3 in these compounds is preferably alkyl, alkenyl or alkoxy having up to 7 C atoms.
Preferred are mixtures comprising one or more compounds of the formulae IIIb, IIIc and IIIf, preferably those in which L1 and/or L2 is F.
Also preferred are mixtures comprising one or more compounds of formula IIIf and/or IIIg, where L2 is H and L1 is H or F, preferably F.
The individual compounds of formulae A, B, IIa-IIk, II*a-II*s and IIIa to IIIj and their subformulae, or alternatively other compounds which can be used in the mixtures according to the invention or in TN and STN displays, are either known or they can be prepared in a manner similar to that for the known compounds.
The compounds of formula A have low viscosities, in particular low rotational viscosities, and low values for the ratio of the elastic constants K33/K11 and therefore their presence results in short switching times in the displays according to the invention, while the presence of compounds of formula B have high dielectric anisotropy, particularly at elevated concentrations, resulting in reduced viscosity.
Preferred liquid-crystal mixtures comprise one or more compounds of component A, preferably in a proportion of 15% to 75%, more preferably from 20% to 65%. These compounds have a dielectric anisotropy xcex94xcex5xe2x89xa7+3, preferably xcex94xcex5xe2x89xa7+8, more preferably xcex94xcex5xe2x89xa7+12.
Further preferred mixtures comprise
one or more, preferably two to four, compounds of formula A,
one, two or three compounds each of the formulae A-1 and A-3,
one or more, preferably one or two, compounds of formula B,
one or more, preferably two to five, compounds of formula IIIb, IIIc and/or IIIf.
Preferred liquid-crystal mixtures comprise one or more compounds of component B, preferably 25 to 85%. The compounds of group B are distinguished, in particular, by their low values of rotational viscosity xcex31.
Component B preferably comprises one or more compounds of formula IV, 
where
L1 and L2 each, independently of one another, are H or F.
Preferred compounds of formula IV are those in which R4 is alkenyl having from 2 to 7 C atoms, particularly those of the following formulae 
where R3a and R4a each, independently of one another, are H, CH3, C2H5 or n-C3H7, and alkyl is an alkyl group having 1 to 7 C atoms.
Preferred are TN and STN displays according to the invention in which the liquid-crystal mixture comprises at least one compound of the formulae IV-1 and/or IV-3 in which R3a and R4a each have the same meaning, and displays in which the liquid-crystal mixture comprises at least one compound of formula IV-5.
In a further preferred embodiment, the mixtures according to the invention comprise one or more compounds of formula IV-6.
Component B preferably further comprises compounds selected from the two-ringed compounds of the formulae V-1 to V-9, 
and/or one or more compounds selected from the three-ringed compounds of the formulae V-10 to V-27, 
and/or one or more compounds selected from the four-ringed compounds of formulae V-28 to V-34, 
where R6 and R7 have the meanings specified for Ra in formula A, and L is H or F.
Preferred are compounds of the formulae V-25 to V-31, where R6 is alkyl and R7 is alkyl or alkoxy, especially alkoxy, each having from 1 to 7 C atoms. Also preferred are compounds of formula V-25, V-28 and V-34, where L is F.
R6 and R7 in the compounds of formulae V-1 to V-34 preferably are straight-chain alkyl or alkoxy having from 1 to 12 C atoms.
Preferred are mixtures according to the invention which comprise one or more compounds of formula B-3a and/or B-5a. 
where R3a is H, CH3, C2H5 or n-C3H7 and alkyl is an alkyl group having 1 to 7 C atoms.
Preferably, the mixtures comprise 2-25 wt %, more preferably 2-15 wt % of compounds of formula B-5a.
The liquid-crystalline mixtures optionally comprise an optically active component C in such an amount that the ratio between layer thickness (spacing of the substrates) and natural pitch of the chiral nematic liquid-crystal mixture is greater than 0.2. This component can be selected by those skilled in the art from a multiplicity of chiral dopants, some of which are commercially available, e.g. cholesteryl nonanoate, S-811, S-1011, S-2011 from Merck KGaA, Darmstadt and CB15 (BDH, Poole, UK). The choice of dopants is not critical per se.
The proportion of the compounds of component C is preferably 0 to 10%, more preferably 0 to 5%, and particularly preferably 0 to 3%.
The mixtures according to the invention may also optionally comprise up to 20% of one or more compounds having a dielectric anisotropy of  less than xe2x88x922 (component D).
If the mixtures comprise compounds of component D, these are preferably one or more compounds containing the structural element 2,3-difluoro-1,4-phenylene, e.g. compounds according to DE-A-38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Preferred are tolanes containing said structural element according to International Patent Application PCT/DE 88/00133.
Further known compounds for component D are, for example, derivatives of the 2,3-dicyanohydroquinones or cyclohexane derivatives containing the structural element 
according to DE-A 32 31 707 and DE-A 34 07 013.
Preferably, the liquid crystal displays according to the invention do not comprise any compounds of component D.
The term xe2x80x9calkenylxe2x80x9d in the meaning of Ra, Rb, Rbb, R2, R3, R4, R5, R6 and R7 encompasses straight-chain and branched alkenyl groups, especially the straight-chain groups. Preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl, and C7-6-alkenyl, especially C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl.
Examples of preferred alkenyl groups include vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to five carbon atoms are generally preferred.
Further preferred embodiments relate to liquid-crystal mixtures according to the invention which
additionally comprise one or more, preferably one, two or three, heterocyclic compounds of formula Va and/or Vb, 
where
R6 and R7 have the above-specified meanings
and
Y is F or Cl,
have the proportion of the compounds from Va and/or Vb of preferably 2 to 35%, more preferably 5 to 20%,
additionally comprise one or more, preferably one, two or three, tolane compounds of the formulae T2a, T2b and/or T2c, 
where R6 and R7 have the above-specified meanings.
The proportion of the compounds of T2a, T2b and/or T2c is preferably 2 to 20%, more preferably 4 to 12%. Preferably, the mixture according to the invention comprises two or three compounds of the formulae T2a and/or T2b.
In preferred embodiments, the mixtures comprise
at least a compound of formula A-1
at least one compound of formula IIb 
at least one compound of formula IIj;
at least one tolane compound of formula T2c;
at least two, especially three compounds of formula A;
at least two, especially three compounds of formula B;
at least one compound of formula T2a;
at least one compound of formula T2b;
at least one compound of the following formula, 
where L1 is H or F;
at least 2.5 wt % of one or more compounds of formula IV-5;
5-30 wt %, preferably 10-25 wt %, of one or more compounds of formula A;
5-30 wt %, preferably 5-20 wt %, of one or more compounds of formula B; and
at least three homologues of the compounds of formula A, where Ra preferably is C2H5, n-C3H7 or n-C5H11.
Preferred homologues of the compounds of formula A are: 
Further preferred embodiments relate to liquid crystal mixtures which
comprise a total of three to six compounds of formulae A and B, the proportion of these compounds in the overall mixture is 25 to 65%, in particular 30 to 55%,
comprise more than 20% of compounds having a dielectric anisotropy xcex94xcex5xe2x89xa7+12.
The mixtures according to the invention are distinguished, particularly when used in TN and STN displays having high layer thicknesses, by very low total switching times (tges=ton+toff).
The liquid crystal mixtures used in the STN and TN cells according to the invention are dielectrically positive with xcex94xcex5xe2x89xa71. Preferred are liquid crystal mixtures having xcex94xcex5xe2x89xa73, particularly those having xcex94xcex5xe2x89xa75.
The liquid crystal mixtures according to the invention exhibit useful values for the threshold voltage V10/0/20 and for the rotational viscosity xcex31. If the optical path difference dxc2x7xcex94n has a predetermined value, the value of the layer thickness d is defined by the optical anisotropy xcex94n. Particularly for relatively high values of dxc2x7xcex94n, the use of liquid crystal mixtures according to the invention having a relatively high value of the optical anisotropy is preferred, since a relatively small value of d can then be chosen, leading to more favorable values for switching times. However, even those liquid crystal displays according to the invention which comprise liquid crystal mixtures according to the invention having smaller values for An are characterized by advantageous values for switching times.
The liquid crystal mixtures according to the invention are further characterized by advantageous values of the slope of the electro-optical characteristic curve and can be operated at high multiplex rates, especially at temperatures above 20xc2x0 C. Moreover, the liquid crystal mixtures according to the invention exhibit high stability and favorable values for electrical resistance and the threshold voltage frequency dependence. The liquid crystal displays according to the invention have a wide operating temperature range and good angular dependence of contrast.
The configuration of the liquid crystal display elements according to the invention, comprising polarizers, electrode baseplates and electrodes surface-treated in such a way that the preferential alignment (director) of the liquid crystal molecules adjoining each of these is usually twisted with respect to one another by an amount of 160xc2x0 to 720xc2x0 from one electrode to the other, which corresponds to the design customary for such display elements. In this context, the term xe2x80x9ccustomary designxe2x80x9d is to be interpreted broadly and also encompasses all alterations and modifications of the TN and STN cell, including matrix display elements, and display elements comprising additional magnets.
The surface tilt angle at the two substrates can be identical or different. Identical tilt angles are preferred. Preferred TN displays have pre-tilt angles between the longitudinal axis of the molecules at the surface of the substrates and the substrates of 0xc2x0 to 7xc2x0, preferably 0.01xc2x0 to 5xc2x0, and more preferably 0.1 to 2xc2x0. In the STN displays the pre-tilt angle is preferably 1xc2x0 to 30xc2x0, more preferably 1xc2x0 to 12xc2x0 and especially 3xc2x0 to 10xc2x0.
The twist angle of the TN mixture in the cell is 22.5xc2x0 to 170xc2x0, preferably 45xc2x0 to 130xc2x0 and more preferably 80xc2x0 to 115xc2x0. In the cell, the twist angle of the STN mixture, from alignment layer to alignment layer, is preferably 100xc2x0 to 600xc2x0, more preferably 170xc2x0 to 300xc2x0, and especially 180xc2x0 to 270xc2x0.
The preparation of the liquid crystal mixtures according to the invention is performed in a manner customary per se. As a guide, the desired quantity of the components used in a smaller amount is dissolved in the components constituting the main constituent, expediently at elevated temperature. Alternatively it is possible to mix solutions of the components in an organic solvent, e.g. in acetone, chloroform or methanol, and to remove the solvent again after thorough mixing, for example by distillation.
The dielectrics may further comprise additives known to those skilled in the art and described in the literature. For example, 0-15% of pleochroitic dyes can be added.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by weight.
The entire disclosure of all applications, patents and publications, cited above or below, and of corresponding German application No. 10060745.4, filed Dec. 7, 2000 is hereby incorporated by reference.
In the present application and in the following examples, the structures of the liquid crystal compounds are given as acronyms, the transformation into chemical formulae being defined by tables A and B. All the radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n or m C atoms. The alkenyl radicals are in the trans configuration. Coding according to table B is self-explanatory. Table A only lists the acronym for the parent structure.
In individual cases, the parent structure acronym is followed, separated therefrom by a dash, by a code, given in the table below, for the substituents R1*, R2*, L2*, L2* and L3*.
The TN and STN displays preferably comprise liquid-crystalline mixtures which are composed of one or more compounds from tables A and B.